AN607

4

Rev. 1.7

2.1. Humidity Sensor Technology

Figure 6 illustrates the humidity sensor configuration. The dielectric layer is exposed to ambient air through the

package opening. Humidity in the air is exchanged with the dielectric material until equilibrium is reached. The

permittivity of the dielectric material is proportional to the amount of moisture it has absorbed. The capacitance

increases as the ambient air becomes more humid and decreases as the ambient air becomes less humid. If the

Si70xx is used at the dew point and condensation occurs on the sensor, the on-chip heater can be activated to dry

the sensor and restore operation once the sensor is above the dew point.

Figure 6. Humidity Sensor Configuration

2.2. Temperature Sensor

The Si70xx parts have a very accurate (as good as ±0.3 °C over the full –40 to +125 °C temperature range)

temperature sensor integrated into the device. The Si70xx family of devices is designed to consume very little

power and does not artificially heat the temperature sensor unless, of course, the heater is turned on. The thermal

paddle on the bottom of the chip is electrically connected to ground inside the chip. The paddle can be soldered to

a ground plane on the PCB which can in turn be extended beyond the chip or can be left floating. In either case, the

ground pad is the thermal input to the on-chip temperature sensor. Care must be taken to isolate the chip from

thermal sources in the system in which it is included. Refer to the section on PCB layout guidelines for more details

on PCB layout and system design considerations to maximize sensor performance.

Digital Output

Er increases as moisture is absorbed

A/D

Moisture

from

Humid Air